Image forming apparatus

ABSTRACT

Provided is an image forming apparatus that forms an image on a recording material, the apparatus including: a first helical gear and a second helical gear that are engaged with each other; and a driving portion that applies a driving force to the first helical gear, wherein at least one of the first helical gear and the second helical gear is a helical gear in which torsional rigidity in a tooth width direction of one side end in a width direction of the gear is larger than torsional rigidity in a tooth width direction of the other side end, and wherein a twist direction of helical teeth and a rotational direction of the first helical gear due to the driving portion are set such that the other side end is engaged earlier than the one side end.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an image forming apparatus such as acopying machine or a printer equipped with a function of forming animage on a recording material such as a sheet.

Description of the Related Art

Japanese Patent Laid-Open No. 9-230657 discloses a configuration inwhich an annular rib is disposed between a central portion and a toothsurface of a gear and the tooth surface and the annular rib are disposedat an interval so as not to come in contact with each other. Accordingto such a configuration, since the tooth surface and the annular rib donot contact with each other, phenomena are suppressed in which a portionof the tooth surface coming in contact with the annular rib is deformedby shrinkage during molding and thus accuracy of the tooth surfacedeteriorates.

By this configuration, it is considered that variation in a positionoccurs due to a rotation fluctuation or vibration of an image preparingportion caused by a rotation fluctuation or vibration occurring at agear engagement cycle and thus a periodic band-like uneven densitycalled a banding image is prevented.

However, the invention disclosed in Japanese Patent Laid-Open No.9-230657 does not cope with reduction in size of a gear and modules forthe purpose of miniaturization of an apparatus body in recent years. Itis difficult to make the modules smaller in the case of reducing thesize of the gear. This reason is that stress applied to a tooth root ofthe gear rises when the size of the module becomes smaller.

Under these circumstance, the inventors paid attention to the fact thata portion of an arm formed between the tooth surface and a rotationsupport portion is disposed at the center of a tooth width direction inthe configuration illustrated in FIG. 3 of Japanese Patent Laid-Open No.9-230657. The inventors found that it is possible to reduce the size ofthe module and to lower the stress applied to the tooth root of the gearby changing the arrangement of the arm.

SUMMARY OF THE INVENTION

The invention is to provide an image forming apparatus capable ofsuppressing stress concentration on a tooth root of a gear.

An image forming apparatus that forms an image on a recording materialincludes: a first helical gear and a second helical gear that areengaged with each other; and a driving portion that applies a drivingforce to the first helical gear, wherein at least one of the firsthelical gear and the second helical gear is a helical gear in whichtorsional rigidity in a tooth width direction of one side end in a widthdirection of the gear is larger than torsional rigidity in a tooth widthdirection of the other side end, and wherein a twist direction ofhelical teeth and a rotational direction of the first helical gear dueto the driving portion are set such that the other side end is engagedearlier than the one side end.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating schematically animage forming apparatus according to the invention.

FIG. 2A is a schematic diagram illustrating a state where motors areconnected to a photosensitive drum and an intermediate belt unit,respectively, and FIG. 2B is a schematic diagram of a drivingconfiguration of a developing device.

FIG. 3 is a schematic diagram of a gear arrangement in the drivingconfiguration of the developing device illustrated in FIG. 2B.

FIGS. 4A and 4B are perspective views illustrating a developing motorgear and a developing reduction gear in detail.

FIG. 5 is a cross-sectional view of the developing motor gear and thedeveloping reduction gear.

FIGS. 6A to 6F are perspective views illustrating calculation results ofa contact state of teeth, respectively.

FIGS. 7A and 7B are numerical value-attached perspective viewsillustrating maximum stress in a developing reduction gear andcalculation results of occurrence points of the maximum stress.

FIG. 8 is a cross-sectional view of a developing motor gear and adeveloping reduction gear according to a second embodiment.

FIG. 9 is a cross-sectional view of a developing motor gear and adeveloping reduction gear according to a third embodiment.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, with reference to the drawings, embodiments of theinvention will be exemplarily described in detail. However, dimensions,materials, shapes, and relative positions of components described in theembodiments are appropriately changed depending on structures andvarious conditions of apparatuses to which the invention is applied andtherefore the scope of the invention is not intended to be limitedthereto unless otherwise particularly specified. In each of thedrawings, components denoted by the same reference numerals have thesame structure or operation, and the duplication description thereofwill not be appropriately presented.

[First Embodiment]

FIG. 1 is a schematic cross-sectional view illustrating schematically animage forming apparatus 50 according to the invention. In the followingdescription, each of the stations denoted by reference numerals with Y,M, C, and K means member for yellow, magenta, cyan, and black, and thesemembers will be described below by reference numerals without signs ofY, M, C, and K. The image forming apparatus 50 illustrated in FIG. 1 isan example of a full-color image forming apparatus (complex machinehaving all of copying machine, printer function, and FAX function). InFIG. 1, the image forming apparatus 50 has a plurality of image formingstations (four image forming stations in this embodiment) which aretransversely juxtaposed with each other in an image forming apparatusbody (hereinafter, referred to as an “apparatus body 50A”).

Each of the stations includes a drum-like electrophotographicphotosensitive drum (referred to as a “photosensitive drum 10” in thisembodiment) as an “image bearing member”. In this embodiment, thephotosensitive drums 10 sequentially bear color images of a yellow (Y)component, a magenta (M) component, a cyan (C) component, and a black(K) component, respectively. These photosensitive drums 10 are rotatablydriven at a predetermined process speed in an arrow direction “A”(counterclockwise direction) by a drum motor which is not illustrated inthe drawing.

For example, a charging device 11, a scanner unit 12, a developingdevice 13, an intermediate belt unit 14, and a cleaning device 15 aresequentially disposed around each of the photosensitive drums 10according to a rotational direction of the photosensitive drum 10. Thecharging device 11 (charging portion) is configured to uniformly chargethe surface of the photosensitive drum 10. The scanner unit 12 (exposureportion) is configured to irradiate the photosensitive drum 10 with alaser beam based on image information and form an electrostatic image onthe photosensitive drum 10.

The developing device 13 as a “developing portion” is configured todevelop the electrostatic image formed on the surface of thephotosensitive drum 10 with a toner and generate a developer image(toner image). The intermediate belt unit 14 (electrostatic transferportion) is configured to transfer the toner image on the photosensitivedrum 10 onto a sheet. The cleaning device 15 (cleaning portion) isconfigured to remove a transfer residual toner remaining on the surfaceof the photosensitive drum 10 after the transfer.

Hereinafter, the image forming station for yellow (Y) out of four colorswill be described as an example. A photosensitive drum 10Y is uniformlysubjected to a charging treatment by a charging device 11Y during arotation process so as to have predetermined polarity and potential.Then, the photosensitive drum 10Y is exposed to light by a laser scanner12Y, whereby an electrostatic image of image information is formed onthe photosensitive drum 10Y.

Next, the electrostatic image formed on the photosensitive drum 10Y isvisualized by a developing device 15Y 13Y and thus a toner image isformed on the photosensitive drum 10Y. Subsequently, the toner imageformed on the photosensitive drum 10 is transferred onto theintermediate belt unit 14 by a primary transfer roller 16Y. Thereafter,the toner image on the intermediate belt unit 14 is transferred onto asheet or other output objects by a secondary transfer roller 17. Similarprocesses are performed on the image forming stations for other threecolors (magenta (M), cyan (C), and black (K)).

[Driving device]

A driving device of an image preparing portion which drives thephotosensitive drum drums 10, the intermediate belt unit 14, and thedeveloping device devices 13 equipped with a driving transmission device(driving force transfer unit), which is a feature of the invention, willbe described below.

FIG. 2A is a schematic diagram illustrating a state where motors 100 and101 are connected to the photosensitive drum drums 10 and theintermediate belt unit 14, respectively. As illustrated in FIG. 2A, thephotosensitive drums 10Y, 10M, and 10C are driven by the motor 100, andthe photosensitive drum 10K and the intermediate belt unit 14 are drivenby the motor 101.

FIG. 2B is a schematic diagram illustrating a state where a motor 102 isconnected to the developing device devices 13. As illustrated in FIG.2B, the developing devices 13Y, 13M, 13C, and 13K are driven by themotor 102.

FIG. 3 is a schematic diagram of a gear arrangement in a drivingconfiguration of the developing device devices 13 illustrated in FIG.2B. As illustrated in FIG. 3, the developing device 13 is devices 13 aredriven by a developing drive gear gears 103 provided coaxially with adrive input position. A DC brushless motor is often used as the motor102, which generally has a rotation speed from about 2000 to 3000 rpm interms of efficiency.

The rotation speed of the developing device devices 13 to be often usedis about 100 to 500 rpm, thereby being reduced by a gear ratio between adeveloping reduction gear 104, a developing motor gear 105, and thedeveloping drive gear gears 103. In the embodiment shown in FIG. 3, asmaller gear 104A rotates coaxially with the developing reduction gear104 and is configured to engage with the developing drive gear 103M andthe developing drive gear 103C. The diameter of the smaller gear 104A issmaller than the diameter of the developing reduction gear 104. As inthis configuration, in a case where a plurality of rotating objects isrotated by one motor, a large load is concentrated on the developingreduction gear 104 compared with a configuration in which one rotationobject is rotated by one motor.

FIG. 4A is a perspective view illustrating the developing motor gear 105and the developing reduction gear 104 in detail. FIG. 4B is a view asseen from the back in FIG. 4A. Referring to FIGS. 4A and 4B, the shapesof the developing motor gear 105 and the developing reduction gear 104corresponding to the driving gear of this embodiment will be describedbelow in detail.

The motor 102 is provided as a “driving portion” which drives thedeveloping motor gear 105 of the developing device devices 13. Thedriving force of the motor 102 is transmitted to the developing devicedevices 13 through a driving transmission portion. The developing motorgear 105 as a “first helical gear” and the developing reduction gear 104as a “second helical gear” are disposed to come in contact with eachother, and the driving force is transmitted to the developing reductiongear 104 from the developing motor gear 105.

In the case of being viewed from the above in FIG. 4A, since thedeveloping motor gear 105 has teeth which are cut in a direction from alower left toward an upper right, it is formed by right-twisted helicalteeth. In addition, since the developing reduction gear 104 has teethwhich are cut in a direction from a lower right toward an upper left, itis formed by left-twisted helical teeth. In this way, the helical gearscoming in contact with each other can be obtained by a combination ofthe right-twisted teeth and the left-twisted teeth in a reversedirection.

As illustrated in FIG. 4A, the developing motor gear 105 is formed insuch a manner of being directly subjected to gear cutting together witha metallic driving shaft 102X of the motor 102 as a “motor” whichgenerates a driving force. Therefore, the developing motor gear 105 istotally formed of a metal.

As illustrated in FIG. 4B, the developing reduction gear 104 is engagedwith the developing motor gear 105. The developing reduction gear 104includes a rim 104c, which is formed of a resin and has an outercircumference formed with teeth, a boss 104d, which is the center ofrotation of the rim 104c (simultaneously, forming the center of rotationof the gear), and a web 104e through which the rim 104c and the boss104d are connected to each other. The developing reduction gear 104 maybe formed of a resin.

In addition, a rib ribs 104f and a rib 104g protrude from the face ofthe web 104e (projecting from the web 104e in a tooth width direction M,discussed further below). As can be seen in FIG. 4, a projectingdimension of the boss 104d from the web 104e is larger than a projectingdimension of each of the ribs 104f from the web 104e. The rib ribs 104fradially extends extend in a radial fashion (in a radial ray fashion)(in a direction separated away from the boss 104d) from the boss 104dfor the purpose of reinforcement of the developing reduction gear 104.The rib 104g is concentrically disposed with respect to the boss 104d.In a direction of the diameter of the developing reduction gear 104, adistance between the boss 104d and the rib 104g is larger than adistance between the rib 104g and the rim 104c. The rib 104f is ribs104f are disposed with a predetermined distance from the rim 104c so asto prevent tooth-face accuracy from deteriorating due to shrinkageduring molding and is are formed not to come in contact with the rim104c.

FIG. 5 is a cross-sectional view of the developing motor gear 105 andthe developing reduction gear 104. The web 104e is provided on a frontside 104a of the developing reduction gear 104. For this reason, a toothwidth direction M of the web 104e is positioned at a left end deviatedfrom a center M1 in the a tooth width direction M. Herein, the toothwidth direction M refers to a thickness direction of the gear.

Therefore, a gradient of torsional rigidity in the tooth width directionM is formed to be large at the front side 104a of the developingreduction gear 104 and to be small at a rear side 104b thereof. That is,the torsional rigidity of developing reduction gear 104 in the toothwidth direction M becomes gradually smaller from the front side 104a(one side) toward the rear side 104b (the other side) in the tooth widthdirection M. For this reason, the developing reduction gear 104 refersto a helical gear in which the torsional rigidity in the tooth widthdirection M of the front side 104a (one side end) in the tooth widthdirection M of the developing reduction gear 104 is larger than thetorsional rigidity in the tooth width direction of the rear side 104b(the other side end).

In other words, the torsional rigidity in the tooth width direction Mbecomes gradually smaller from a side where closer to the web 104e iscloser to in the tooth width direction M toward a side where fartherfrom the web 104e is not closer to in the tooth width direction M. Forthis reason, it is said that torsional rigidity at the side wherefarther from the web 104e is not closer to in the tooth width directionM is smaller than the torsional rigidity at the side where closer to theweb 104e is closer to in the tooth width direction M. At least one ofthe developing motor gear 105 and the developing reduction gear 104 maybe configured in this manner.

Returning back to FIGS. 4A and 4B, the description will be made below.The developing motor gear 105 rotates in a direction indicated by anarrow A, and the developing reduction gear 104 engaged with thedeveloping motor gear 105 rotates in a direction indicated by an arrowB. The A helical gear has a property that comes in contact with theother gear to be engaged from an advancing side in the rotationaldirection.

That is, the a helical gear sequentially comes in contact with the othergear to be engaged from an advancing helical tooth in the advancingdirection of each rotating helical teeth. That is, since the developingmotor gear 105 is right-twisted and thus rotates in the directionindicated by the arrow A, a rear end 105X2 of helical teeth 105X rotatesengages with a helical tooth 104X of the developing reduction gear 104earlier than a front end 105X1 thereof in the direction indicated by thearrow A. In addition, since the developing reduction gear 104 isleft-twisted and thus rotates in the direction indicated by the arrow B,a rear end 104X2 of helical teeth 104X rotates engages with a helicaltooth 105X of the developing motor gear 105 earlier than a front end104X1 thereof in the direction indicated by the arrow B. Accordingly,the developing motor gear 105 and the developing reduction gear 104 comein contact with the other gear to be engaged from the rear ends 105X2and 104X2 advancing in the advancing direction, respectively.

In the configuration of this embodiment, the direction of the helicalteeth is set such that the contact occurs from the rear side 104b havingthe small torsional rigidity. That is, a twist direction of the helicalteeth and the rotational direction of the developing motor gear 105 dueto the motor 102 are set such that the developing motor gear 105 and thedeveloping reduction gear 104 are engaged with each other in such amanner that the teeth come in contact with each other at the side (theother side in the tooth width direction) (the other side end) having thesmall torsional rigidity earlier than the side (one side in the toothwidth direction) (one side end) having the large torsional rigidity.

A simulation experiment is performed to observe the contact state of theteeth of this embodiment configured as described above and to calculatethe maximum value of tooth root stress. The simulation experiment isperformed using Abaqus which is versatile software for non-linearstructure analysis. The developing motor gear 105 is a rigid body andthe developing reduction gear 104 is an elastic body having a Young'smodulus of 2200 MPa. A module of the gear is 0.4, a twist angle is 20°,a pressure angle is 20°, the number of teeth of the developing motorgear 105 is 11, the number of teeth of the developing reduction gear 104is 86, and a driving load is 0.8 N·m. In this way, the number of teethof the developing motor gear 105 is set to be smaller than the number ofteeth of the developing reduction gear 104.

FIGS. 6A to 6F are perspective views illustrating calculation results ofthe contact state of the teeth when the developing reduction gear 104rotates in the direction indicated by an arrow B and an arrow C. Inorder to make it easy to see the contact state, only the tooth surfaceof the developing motor gear 105 is illustrated. In addition, thedeveloping reduction gear 104 illustrated in FIGS. 6A to 6F looks like aspur gear in external appearance, but has the left-twisted helical teethwhich are left-twisted with respect to the axial line as described abovein fact. FIGS. 6A to 6F are enlarged diagrams of the actual helicalteeth, respectively.

In FIGS. 6A to 6F, a contact portion is indicated by a black-paintedportion K. The developing reduction gear 104 rotates in the directionindicated by the arrow B in this order of FIGS. 6A to 6C. As the helicalteeth 105X of the developing motor gear 105 rotate, the developing motorgear 105 continues to come in sequential contact with the developingreduction gear 104 from the rear end 105X2 side which is the advancingside of the helical teeth. Then, the contact area between the developingmotor gear 105 and the developing reduction gear 104 shifts from therear end 104X2 of the developing reduction gear 104 to the front end104X1. In addition, it is understood that three helical teeth 105X ofthe developing motor gear 105 always come in contact with the developingreduction gear 104 during the rotation.

FIGS. 6D to 6F are illustrated in comparison with this embodiment andare perspective views illustrating calculation results of a contactstate of teeth when the developing reduction gear 104 rotates in thedirection indicated by an arrow C which is a direction reverse to thedirection illustrated in FIGS. 6A to 6C, respectively. Since the helicalteeth 105X of the developing motor gear 105 rotate in a reversedirection, the advancing direction of the helical teeth is also reverseand thus the developing motor gear 105 continues to come in sequentialcontact with the developing reduction gear 104 from the front end 105X1side. Then, the number of the helical teeth 105X of the developing motorgear 105, which always come in contact with the developing reductiongear 104 during the rotation, is reduced to two.

Comparing these two examples with each other, in the rotationaldirection of this embodiment, since the developing motor gear 105 comesin contact with the developing reduction gear 104 from the rear side104b having the small torsional rigidity, and the developing motor gear105 comes in contact with a deformable portion of the developingreduction gear 104, the number of the teeth of the developing motor gear105 coming in contact with the developing reduction gear 104 at alltimes increases. Meanwhile, in the rotation in the reverse direction,since the developing motor gear 105 comes in contact with the developingreduction gear 104 from the front side 104a having the large torsionalrigidity, and the developing motor gear 105 comes in contact with ahardly deformable portion of the developing reduction gear 104, thenumber of the teeth of the developing motor gear 105 coming in contactwith the developing reduction gear 104 at all times reduces.

FIGS. 7A and 7B are numerical value-attached perspective viewsillustrating maximum stress and calculation results of occurrence pointsof the maximum stress in the developing reduction gear 104,respectively. It indicates that stress becomes gradually higher in theorder from a gray-painted portion K2 to a black-painted portion K1. FIG.7A illustrates the calculation results when the developing reductiongear 104 rotates in the rotational direction (direction indicated by thearrow B) of this embodiment corresponding to FIGS. 6A to 6C. FIG. 7Billustrates the calculation results when the developing reduction gear104 rotates in the rotational direction (direction indicated by thearrow C) corresponding to FIGS. 6D to 6F, which is reverse to therotational direction of this embodiment.

A stress value is expressed by the maximum principal stress. Even in anycase, the maximum stress occurs in a tooth root in the vicinity of thefront side 104a having the large torsional rigidity. When the maximumstress value in this embodiment (see FIG. 7A) is 1, the The maximumstress value during the rotation in the reverse direction (see FIG. 7B)becomes is 2.3 times larger than the maximum stress value of thisembodiment (see FIG. 7A) during the rotation in the direction indicatedby the arrow B.

In this embodiment (see FIG. 7A), since the contact number of the teeth,that is, the contact area to which the load is applied becomes largerwhen the developing reduction gear 104 rotates in the directionindicated by the arrow B, the stress value becomes smaller so as torelatively reduce the amount of deformation of each tooth and themaximum stress also becomes smaller, which is 84.5 MPa. Whereas, in thecomparative example (see FIG. 7B), since the contact number of theteeth, that is, the contact area to which the load is applied becomessmaller when the developing reduction gear 104 rotates in the directionindicated by the arrow C, the stress value becomes larger so as torelatively increase the amount of deformation of each tooth and themaximum stress also becomes larger, which is 194 MPa.

According to this embodiment, when high loads are transmitted with asmall module, since the rigidity increases to ensure strength and thusdeterioration in accuracy of the tooth surface is not caused by theshrinkage during molding, it is possible to provide a drivingconfiguration in which a high-quality image not having a banding imagecan be output.

[Second Embodiment]

FIG. 8 is a cross-sectional view of a developing motor gear 105 and adeveloping reduction gear 104 according to a second embodiment. Aconfiguration of the second embodiment is the same as similar to that ofthe first embodiment except that a gradient of torsional rigidity in atooth width direction M of the developing reduction gear 104 isprovided, and the description of features of the second embodiment thathave the same configuration as the first embodiment will not bepresented.

The thickness of a rim 104c of the developing reduction gear 104 becomesgradually thinner from a front side 104a (one side) toward a rear side104b (the other side) in the tooth width direction M. For this reason,the gradient of the torsional rigidity in the tooth width direction M isformed to be large at the front side 104a and to be small at the rearside 104b. Thus, the torsional rigidity in the tooth width direction Mof the developing reduction gear 104 becomes gradually smaller from thefront side 104a (one side) toward the rear side 104b (the other side) inthe tooth width direction M. In other words, the torsional rigidity inthe tooth width direction M becomes gradually smaller from a thick sideof the rim 104c toward a thin side of the rim 104c. For this reason, itis also considered that the torsional rigidity at the thin side of therim 104c is smaller than the torsional rigidity at the thick side of therim 104c.

[Third Embodiment]

FIG. 9 is a cross-sectional view illustrating a developing motor gear105 and a developing reduction gear 104 according to a third embodimentin detail. A configuration of this embodiment is the same as similar tothat of the first embodiment except that a gradient of torsionalrigidity in a tooth width direction M of the developing reduction gear104 is provided, and the description of features of the third embodimentthat have the same configuration as the first embodiment will not bepresented.

In this embodiment, the developing reduction gear 104 is formed with aweb 104e between a boss 104d and a rim 104c. The web 104e is disposedsubstantially at the center in the tooth width direction M of thedeveloping reduction gear 104. The web 104e is formed in a disk-likeplate shape around the boss 106 104d.

On the premise of this configuration, a rib 104f radially extends fromthe boss 104d (this is the same as the configuration in FIG. 4B) andprotrudes toward a front side 104a from the web 104e (this is differentfrom the configuration in FIG. 4B) at the same time. In this way, therib 104f is disposed at only the front side 104a (one side) in the toothwidth direction M. For this reason, a gradient of torsional rigidity inthe tooth width direction M is formed to be large at the front side 104aand to be small at a rear side 104b.

Thus, the torsional rigidity in the tooth width direction M of thedeveloping reduction gear 104 becomes gradually smaller from the frontside 104a (one side) toward the rear side 104b (the other side) in thetooth width direction M. In other words, the torsional rigidity in thetooth width direction M becomes gradually smaller from a side disposedwith the rib 104f toward a side not disposed with the rib 104f. For thisreason, it is also considered that the torsional rigidity in the toothwidth direction M at the side not disposed with the rib 104f is smallerthan the torsional rigidity in the tooth width direction M at the sidedisposed with the rib 104f.

According to any one configuration of the first to third embodiments, itis possible to suppress stress concentration on a gear tooth root evenwhen the module is reduced compared to the related art.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-221347, filed Oct. 30, 2014, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus that forms an image ona recording material, the image forming apparatus comprising: a firstphotosensitive drum; a second photosensitive drum; a first driving motorconfigured to generate a driving force to be transferred to the firstphotosensitive drum and the second photosensitive drum; a first gear; afirst developing portion connected to the first gear, the firstdeveloping portion being configured to develop an electrostatic image onthe first photosensitive drum with a toner and rotate in a case where adriving force is transferred to the first gear; a second gear; a seconddeveloping portion connected to the second gear, the second developingportion being configured to develop an electrostatic image on the secondphotosensitive drum with a toner of color different from the toner ofthe first developing portion and rotate in a case where a driving forceis transferred to the second gear; and a driving force transfer unitconfigured to transfer a driving force to the first gear and the secondgear, the driving force transfer unit including: (A) a first helicalgear including a plurality of teeth; and (B) a second driving motorconfigured to generate a driving force for rotating the first helicalgear and connected to the first helical gear to transmit the drivingforce of the second driving motor to the first helical gear; and (C) asecond helical gear that is engaged and a third gear which transfer thedriving force of the second driving motor to the first gear and thesecond gear, the second helical gear configured to engage with the firsthelical gear, the second helical gear having a first side and a secondside with respect to an axis direction of the second helical gear thethird gear rotating coaxially with the second helical gear andconfigured to engage with the first gear and the second gear, each ofthe third gear and the second helical gear having a diameter, thediameter of the third gear being smaller than the diameter of the secondhelical gear, the second helical gear including: (a) a rim that hasincluding an outer circumference formed with a plurality of teeth,configured to engage with the plurality of teeth of the first helicalgear, the second helical gear including a first side relative to acenter of the rim in a tooth width direction of the second helical gearand a second side relative to the center of the rim in the tooth widthdirection of the second helical gear; (b) a boss that is a center ofrotation of the rim, and second helical gear; (c) a web through which(i) disposed at the first side and (ii) connecting the rim and thebossof the second helical gear are connected to each other,; (d) aconcentric rib (i) projecting from the web to the second side in thetooth width direction and (ii) being concentric with the boss; and (e) aplurality of radial ribs (i) projecting from the web to the second sidein the tooth width direction, (ii) extending from the boss in a radialfashion so as to connect the boss and the concentric rib, and (iii)disposed apart from the rim in a radial direction of the second helicalgear, wherein the web is disposed so as to be shifted toward the firstside, and the plurality of teeth of the second helical gear isconfigured so that a region of each of the plurality of teeth at thesecond side of the second helical gear engages with the first helicalgear earlier than when another region of each of the tooth teeth at thefirst side of the second helical gear engages with the first helicalgear.
 2. The image forming apparatus according to claim 1, wherein atorsional rigidity of the second side of the second helical gear issmaller than a torsional rigidity of the first side of the secondhelical gear.
 3. The image forming apparatus according to claim 1,wherein a thickness of the rim at the second side of the second helicalgear is less than a thickness of the rim at the first side of the secondhelical gear.
 4. The image forming apparatus according to claim 1,wherein the second helical gear includes a plurality of ribs thatprotrude in the axis direction from a face of the web and extendradially from the boss, such that each rib is exposed to only the secondside of the second helical gear and is not exposed to the first side ofthe second helical gear, and a torsional rigidity of the second side ofthe second helical gear is smaller than a torsional rigidity of thefirst side of the second helical gear.
 5. The image forming apparatusaccording to claim 1, wherein the second helical gear is formed of aresin and the first helical gear is formed of a metal.
 6. The imageforming apparatus according to claim 5, wherein the number of teeth ofthe first helical gear is set to be smaller than the number of teeth ofthe second helical gear.
 7. The image forming apparatus according toclaim 6, further comprising: a wherein the second driving motor thatapplies a driving force to the first helical gear includes a drivingshaft, and wherein the first helical gear is subjected to gear cuttingtogether with a formed by teeth cut directly on the driving shaft of themotor.
 8. The image forming apparatus according to claim 7, furthercomprising: an image bearing member; and a developing portion thatdevelops an electrostatic image on a surface of the image bearing memberwith a toner, wherein the motor that applies the driving force to thefirst helical gear drives the developing portion.
 9. The image formingapparatus according to claim 1, wherein the web has first and secondouter surfaces with respect to the axis direction of the second helicalgear, the first and second outer surfaces of the web being positioned atthe first side of the second helical gear deviated from a center of thesecond helical gear in the axis direction.
 10. An image formingapparatus that forms an image on a recording material, the image formingapparatus comprising: a first helical gear; and a second helical gearthat is engaged with said first helical gear, the second helical gearhaving a first side and a second side with respect to an axis directionof the second helical gear, the second helical gear including aplurality of teeth formed at an outer peripheral surface thereof and awall portion formed at an inner peripheral surface thereof toward arotation center of the second helical gear, the wall portion beingformed continuously along the inner peripheral surface of the secondhelical gear, wherein the wall portion is disposed so as to be shiftedtoward the first side from a center with respect to the axis directionof the second helical gear, and the plurality of teeth of the secondhelical gear is configured so that a region of each of the plurality ofteeth at the second side of the second helical gear engages with thefirst helical gear earlier than when another region of the tooth at thefirst side of the second helical gear engages with the first helicalgear.
 11. The image forming apparatus according to claim 10, wherein thewall portion has first and second outer surfaces with respect to theaxis direction of the second helical gear, the first and second outersurfaces of the wall portion being positioned at the first side of thesecond helical gear deviated from a center of the second helical gear inthe axis direction.
 12. The image forming apparatus according to claim11, wherein the wall portion is disposed at an end portion of the secondhelical gear in the axis direction of the second helical gear.
 13. Theimage forming apparatus according to claim 1, wherein, in a direction ofthe diameter of the second helical gear, no rib is disposed between theconcentric rib and the rim.
 14. The image forming apparatus according toclaim 1, wherein in a direction of the diameter of the second helicalgear, a distance between the boss and the concentric rib is larger thana distance between the concentric rib and the rim.
 15. The image formingapparatus according to claim 1, wherein, in the tooth width direction ofthe second helical gear, a projecting dimension of the boss from the webis larger than a projecting dimension of the plurality of radial ribsfrom the web.
 16. An image forming apparatus that forms an image on arecording material, the image forming apparatus comprising: a firstphotosensitive drum; a second photosensitive drum; a first driving motorconfigured to generate a driving force to be transferred to the firstphotosensitive drum and the second photosensitive drum; a first gear; afirst developing portion connected to the first gear, the firstdeveloping portion being configured to develop an electrostatic image onthe first photosensitive drum with a toner and rotate in a case where adriving force is transferred to the first gear; a second gear; a seconddeveloping portion connected to the second gear, the second developingportion being configured to develop an electrostatic image on the secondphotosensitive drum with a toner of color different from the toner ofthe first developing portion and rotate in a case where a driving forceis transferred to the second gear; and a driving force transfer unitconfigured to transfer a driving force to the first gear and the secondgear, the driving force transfer unit including: (A) a first helicalgear including a plurality of teeth formed thereon; (B) a second drivingmotor configured to generate a driving force for rotating the firsthelical gear and connected to the first helical gear to transmit thedriving force of the second driving motor to the first helical gear; and(C) a second helical gear and a third gear which transfer the drivingforce of the second driving motor to the first gear and the second gear,the second helical gear configured to engage with the first helicalgear, the third gear rotating coaxially with the second helical gear andconfigured to engage with the first gear and the second gear, each ofthe third gear and the second helical gear having a diameter, thediameter of the third gear being smaller than the diameter of the secondhelical gear, the second helical gear including: (a) a rim including anouter circumference formed with a plurality of teeth configured toengage with the plurality of teeth of the first helical gear; (b) a bossthat is a center of rotation of the second helical gear; (c) a web (i)disposed at one side relative to a center of the rim in a tooth widthdirection of the second helical gear and (ii) connecting the rim and theboss; (d) a concentric rib (i) projecting from the web to another sidein the tooth width direction and (ii) being concentric with the boss;and (e) a plurality of radial ribs (i) projecting from the web to theanother side in the tooth width direction, (ii) extending from the bossin a radial fashion so as to connect the boss and the concentric rib,and (iii) disposed apart from the rim, wherein the first helical gearand the second helical gear rotate to transfer the driving force of thesecond driving motor to the first gear and the second gear so that acontact portion between the teeth of the first helical gear and theteeth of the second helical gear shifts from the another side in thetooth width direction to the one side.
 17. The image forming apparatusaccording to claim 16, wherein a twist direction of the teeth of thefirst helical gear, a twist direction of the teeth of the second helicalgear, a rotary direction of the first helical gear, and a rotarydirection of the second helical gear are configured so that an area ofthe another side in the tooth width direction of the second helical gearis contacted earlier than an area of the one side in the tooth widthdirection of the second helical gear.
 18. The image forming apparatusaccording to claim 16, wherein a torsional rigidity of the secondhelical gear in the tooth width direction where the first helical gearand the second helical gear begin to be engaged is smaller than atorsional rigidity of the second helical gear in the tooth widthdirection where the web is disposed.
 19. The image forming apparatusaccording to claim 16, wherein the second helical gear is formed of aresin and the first helical gear is formed of a metal.
 20. The imageforming apparatus according to claim 16, wherein a number of teeth ofthe first helical gear is smaller than a number of teeth of the secondhelical gear.
 21. The image forming apparatus according to claim 16,wherein the second driving motor includes a driving shaft, and whereinthe first helical gear is formed by teeth cut directly on the drivingshaft.
 22. The image forming apparatus according to claim 16, wherein,in a direction of the diameter of the second helical gear, no rib isdisposed between the concentric rib and the rim.
 23. The image formingapparatus according to claim 16, wherein, in a direction of the diameterof the second helical gear, a distance between the boss and theconcentric rib is larger than a distance between the concentric rib andthe rim.
 24. The image forming apparatus according to claim 16, wherein,in the tooth width direction of the second helical gear, a projectingdimension of the boss from the web is larger than a projecting dimensionof the plurality of radial ribs from the web.